Dam spillway gate collapse may have fatal consequences and cause severe structural damage due to flooding, additionally the dam owner will suffer substantial business losses. The repair work required to put a gate back in service can be time consuming, challenging, dangerous and costly. To ensure the reliability of radial gate operation, and depending on the type of trunnion bearing and the structural capacity of the gate arms, the bearing friction should be carefully monitored and gate performance evaluated to confirm the gate’s ability to withstand increases in friction over time. The frequency of monitoring requires careful consideration.
Radial gate arms are normally designed to withstand bending moments from nominal bearing friction. An inappropriate bearing, or a bearing in poor condition, might have friction sufficiently high to cause a gate arm to fail due to the excessive bending moment during gate operation.
An easy and non-invasive way of analysing the condition of the bearing, to ensure safe operation of radial gates where the arms might be prone to increased bending moment, is through friction measurement with the use of strain gauges. This paper briefly presents common radial gate design and some failure modes as a consequence of increased bearing friction, and a method of determining the bearing friction coefficient through strain gauge measurements and experience from the field is presented.
The waters that feed the Nyamwamba River in western Uganda start as meltwater from the glaciers high up in the Rwenzori Mountains. A small scale run-of-river hydropower plant, equipped with a low height tyrolean type intake weir, is now operating just upstream of the town of Kilembe, the first large community along this river. History has seen floods cause realignments of the river through the town and major damage to property and loss of life.
A devastating flood occurred during the design phase for the scheme prior to any construction commencing, which caused loss of life and significant damage to roads, bridges and buildings within the town, including the hospital. Design changes to improve resilience of all riverine connections were made, including relocation of the diversion weir to a stronghold point within the basic protection zone of a natural island. A flood diversion dyke was constructed across one of the river branches that flows around the island, with its alignment, type and height optimised to capture low flows for energy generation while deflecting large flows away from the weir to mitigate flood damage.
Another major flood arrived three months after completion. No damage was sustained which provided confidence in the resilience of the headworks. A major river dredging program contributed to the overall resilience of this reach of river through the town.
This paper describes the challenges for the development of the project site in terms of physical considerations to work with the river, adopting some lessons learned from the pre-construction floods.
Vertical gates and their operating plant are an essential part of dam safety at many dam sites. Apportioning appropriate levels of resilience during the design phase requires a thorough understanding of a gate system as a whole, not only of a single component in isolation.
This paper offers a designer’s perspective on modern engineering design features, materials and practices which can improve gate resilience during onerous operating conditions. This is of particular relevance to gates that are seldom used. Design aspects relating to the capability and limitations of the gate, hoist type, power supply arrangement and control system equipment to work together as a complete system are paramount design considerations in ensuring overall system resilience.
A discussion of the role and duty a hydraulic gate has in a dam safety context is presented. Supporting commentary is offered on appropriate levels of reliability, redundancy and diversity, including a comparison of different gate, bearing and hoist types. The authors draw on their own experience regarding gate design, fabrication and operation from completed and ongoing projects both locally and internationally.
Computational Fluid Dynamics (CFD) is the science of predicting momentum, mass and heat transport, and can aid in design and safety issues for dam resilience in modern settings. Applications of CFD have historically been in the aerospace, automotive and chemical process industries with limited application in the hydraulic engineering field; possibly due to the associated computational intensity that is typically required. However, over the past two decades the cost of computing power has decreased substantially while the processing speed has increased exponentially. These developments have now made the application of CFD in the commercial environment feasible. CFD is particularly valuable in complex flow situations where the outputs required cannot be provided by a traditional hydraulic assessment approach and where there are stakeholder drivers such as service life, insurance cover and safety implications of infrastructure. The need for CFD when these drivers and complex flow situations arise, are demonstrated by means of a case study.
In the case study, CFD was used to investigate the flow patterns and the predicted performance of the outlet pipework from Massingir Dam in Mozambique. Three flow scenarios with appropriate pressure and flow boundary conditions were analysed for the outlet pipework, which included bifurcations for power generation from the main discharge conduits. Specific concerns addressed were, firstly, the possible excessive negative pressure in the region of the offtake for power generation and the potential for cavitation effects and, secondly, unacceptable velocity gradients in the power offtake pipework. Results showed that although some negative pressures were possible in one flow scenario, mitigation measures based on the CFD outputs could be considered and designed before construction.
The implementation of CFD in the above case study displays how risk in design can be reduced to ensure safety issues are addressed effectively.
Flood inundation consequence and emergency evacuation assessment using advanced numerical modelling tools such as HEC-LifeSim is progressively emerging as accepted best practice, due in part to the growing ease in obtaining the necessary datasets and hydraulic numerical modelling results and the increasing computational power readily available to perform analyses. In turn, these tools are being applied to assess dam failure consequence and the effectiveness of emergency response procedures.
An essential resource is an approved Emergency Action Plan (EAP, also known as a Dam Safety Emergency Plan), which describes how dam owners and disaster management groups notify and warn persons at risk of harm during an emergency event. There have been progressive improvements in the effectiveness of EAPs through a series of reviews and lessons learnt from emergency events, legislative and regulatory amendments and general improvements in communications, monitoring, alerts and public awareness. Effectiveness is measured through feedback from training exercises and expert reviews, however a more quantitative measure is not presently available. This limitation can challenge decision makers who need to balance costs associated with emergency preparedness with anticipated reductions in life safety risks.
The paper explores the feasibility of providing a quantitative assessment of the effectiveness of an EAP using advanced consequence modelling (HEC-LifeSim). Using consequence models for two dams in Queensland, EAP effectiveness is assessed for a range of emergency response measures. The accuracy and reliability of the model parameters applied to each simulation and their impact upon the reliability of predictions of potential loss of life (PLL) are analysed and discussed. The feasibility of the approach is discussed and recommendations to be considered for future applications made.
Trustpower’s Mahinerangi Dam in New Zealand’s South Island is a concrete arch and gravity abutment dam built in 1931, subsequently raised in 1946 and strengthened with tie-down anchors in 1961.
This paper discusses a 3D finite element analysis of the dam and the predicted performance of the arch section under Safety Evaluation Earthquake (SEE) loading against identified potential failure modes.
Current guidelines and recent seismic hazard assessments recommend earthquake loadings higher than what was originally accounted for in previous decades. A Comprehensive Safety Review identified stability under SEE loading as a potential deficiency, so a programme of works was commenced to evaluate and better understand the seismic risk by using modern day tools and technology to evaluate the dam against current performance standards.
The final model incorporated the results of extensive laboratory testing, high-resolution LiDAR survey data and dynamic calibration using ambient-vibration monitoring. Motion recordings across the face of the dam during the 2016 Kaikōura earthquake were also used to validate the model. The reservoir has been explicitly modelled together with the opening, closing and sliding of contraction joints and the foundation interface. This allowed the modelling of permanent displacements and the redistribution of loads within the dam under SEE loading, which had been shown to be an important behaviour from the previous stages of analysis.